Fresnel zone plates offer focusing effects that focus radio waves and increase energy density. Accordingly, antenna devices that include Fresnel zone plates in front are known to improve antenna gain in front. For this reason, an antenna device including a Fresnel zone plate can transmit a radio frequency (RF) signal contained in radio waves to a farther point than an antenna device that does not have a Fresnel zone plate.
However, if high antenna gains are obtained in directions other than the front direction, a problem arises that delayed waves are transmitted and received due to multipath effects and the communication performance of the antenna degrades.